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The use of a model to appraise arboviral epizootics is described, and examples are given which demonstrated long-distance wind-dispersal of the biting-midge vector Culicoides brevitarsis from the same endemic source. Vector abundance is characterised by peaks and troughs of abundance, the patterns of which vary between localities and annually. Short term climatic effects can change rapidly the age structure of a population, thus increasing the number of midges that can transmit a virus and profoundly influencing the probabilities of viral transmission. The population dynamics of vector and viral foci are complex, if not chaotic.

Infection of cattle with bluetongue and Douglas viruses was detected on the central and southern coast of New South Wales from January to April 1989. Bluetongue virus infection was found well south of areas of expected occurrence. Evidence is presented to support wind-borne dispersal of infected vectors, Culicoides brevitarsis, southwards from the Hunter Valley.

Perinatal lamb mortality, associated with malformations of the CNS due to Akabane viral infection, occurred in 4 of 9 flocks of ewes lambing on 3 farms between 26 May and 14 November, 1976. Cases were restricted to ewes conceiving prior to the second week of March and lambing between 26 May and 19 July. As judged by seroconversion in sentinel flocks on 2 of the farms, field infection with Akabane virus occurred mainly between mid-February and mid-April. Malformations of the CNS occurred in 42.5%, 51.2%, 100% and 31.0% of the dead lambs examined in the affected flocks respectively. Prevalence in the 4 affected flocks, expressed as the proportion of ewes lambing which delivered at least one malformed foetus, was 6.1%, 8.4%, 88.9% and 5.7% respectively. Lamb mortality due to malformations of the CNS was 7.1%, 5.5%, 92.3% and 5.7% of lambs born. Age-specific prevalence was calculated for 3 of the 4 flocks and 2-year-old ewes accounted for 71.4% and 76.9% of total cases respectively in 2 flocks, whereas in one flock malformations occurred at equivalent frequencies throughout several older age groups. Birthweights of affected lambs were usually significantly lighter than those of unaffected lambs of similar sex and birth-type, and their mean duration of gestation was slightly, and significantly, prolonged. Micrencephaly (88.1% of cases) and hydrocephalus (68.7% of cases) were the outstanding pathological features of the malformations with hydranencephaly, microgyria, porencephaly and attenuation of the spinal cord occurring at much lower frequencies.(ABSTRACT TRUNCATED AT 250 WORDS)

In 1983 an outbreak of Akabane disease occurred in calves in New South Wales between Coolah and Dunedoo at the foothills of the Liverpool Range, from Molong to Oberon in the Blue Mountains and in the Bylong Valley. These areas, at the time of infection of the dams, were in a drought and conditions were unsuitable for the multiplication of C. brevitarsis. In late March meteorological phenomena developed producing air movements favourable for transporting infected midges from the Hunter Valley. A re-examination of the data from the epizootics of 1974 and 1955 showed that similar weather systems had developed. It is possible that the source of infected midges in these epizootics was also the Hunter Valley.